This application claims the priority of application number 199 55 508.7, filed in Germany, Nov. 18, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an internal combustion engine with an exhaust gas turbocharger, and an associated method. Preferred embodiments relate to an exhaust gas turbocharger, which comprises an exhaust gas turbine and a compressor for generating compressed boost air, having an additional, air-driven turbine, which is torsionally connected to the compressor and to which combustion air can be supplied via an adjustable shut-off element, a turbine outlet of the air-driven turbine being connected by a connecting duct to the induction path downstream of the compressor and having a closed-loop and open-chain control unit for generating setting signals which adjust the shut-off element.
An exhaust gas turbocharger for an internal combustion engine is known from German Patent Document DE 42 13 047 A1, whose exhaust gas turbine is driven by the exhaust gas back pressure and drives, via a connecting shaft, a compressor for generating increased boost pressure. In order to improve the transient behavior of the exhaust gas turbocharger, compressed air from a pressure reservoir in the compressor inlet can be fed in via an additional duct to support the run-up of the supercharger to higher peripheral speeds; at the same time, a throttle upstream of the compressor inlet is displaced into the shut-off position in order to prevent the compressed air fed in at a positive pressure from escaping to the atmosphere via the intake line.
By means of this exhaust gas turbocharger, it is in fact possible to increase the supercharger rotational speed even at low-load operating points, at which only a small exhaust gas back pressure is built up, by feeding in additional compressed air. In this way, delays in the build-up of pressure, which may be attributed to a delay in the increase in the supercharger rotational speed due to inertia, can be reduced. This advantage must, however, be purchased at the cost of a high level of structural complication. In particular, it is necessary to ensure a sufficiently high positive pressure in the pressure reservoir, for which purpose an additional compressor, including the drive unit necessary for the compressor, is required.
In order to improve the response behavior of exhaust gas turbochargers, the inertia of the superchargers has been previously reduced by a reduction in size and a lighter design by means, in particular, of reducing the turbine impeller diameter. As a result of the smaller mass moment of inertia of the supercharger rotor, the delay in the build-up of boost pressure is reduced and, correspondingly, a higher engine torque can be built up in a shorter time. The reduction in the impeller volume of the supercharger rotor can, however, lead to a deterioration in the efficiency of the exhaust gas turbocharger. A further problem associated with relatively small exhaust gas turbocharger designs is the high supercharger rotational speeds and, in particular, the large rotational speed range which has to be overcome by the supercharger rotor between lower engine load and full load.
A further exhaust gas turbocharger device of the generic type is revealed in Japanese Patent Document JP 10-315 616 A1. Associated with the exhaust gas turbocharger described in this publication is an additional, air-driven turbine, which is seated on the same drive shaft as the exhaust gas turbine and the compressor and is capable of supplying drive power to the compressor. The air-driven turbine is fed with compressed air from a compressed air reservoir and the expanded air at the turbine outlet of the air turbine is combined with the compressed air from the compressor and subsequently introduced into the air inlet of the internal combustion engine.
The compressed-air driven turbine is switched on in the lower load range, during which an exhaust gas back pressure sufficient for driving the exhaust gas turbine is not yet available or is just being built up, so that the rotor of the exhaust gas turbocharger can be accelerated to higher revolutions despite a low exhaust gas back pressure. Although this appliance makes it possible to build up an exhaust gas turbocharger rotor rotational speed sufficient for a build-up of boost pressure over a wide operating range of the internal combustion engine, an additional compressed air reservoir, including the associated drive units, must again be considered as reservations with respect to the appliance as described in this JP 10-315 616 A1.
The invention is based on the problem of improving the efficiency of an exhaust gas turbocharger by simple means.
This problem is solved, in accordance with the invention, by providing an arrangement of the above referred to type, wherein an additional duct is provided between the compressor inlet or a line section opening into the compressor inlet and the air inlet of the air-driven turbine,
wherein the air supply in the additional duct and in the compressor inlet can be adjusted by the shut-off element, and
wherein in a lower load range, in a case where required boost pressure falls below a threshold value, a setting signal is generated in the closed-loop and open-chain control unit, which setting signal adjusts the shut-off element into a position opening the additional duct and reducing the air supply to the compressor inlet.
This problem is also solved by a method of operating an internal combustion engine having an exhaust gas turbocharger with an exhaust gas turbine and a compressor for generating compressed boost air, having an additional, air-driven turbine, which is torsionally connected to the compressor and to which combustion air can be supplied via an adjustable shut-off element, the turbine outlet being connected to a duct section communicating with the compressor outlet, wherein, in a lower load range, in a case where the required boost pressure falls below a threshold value, at least a partial flow of the induced combustion air is guided via the air-driven turbine.
The novel internal combustion engine comprises an exhaust gas turbocharger which has an additional duct between the compressor inlet and the air inlet of the air-driven turbine, it being possible to adjust the air supply to the additional duct and also to the compressor inlet by a shut-off element, as a function of the operating condition of the internal combustion engine. This embodiment offers the advantage that the supply of combustion air to both the compressor and the air-driven turbine can take place via a common induction duct. The feed to the compressor and/or to the air turbine takes place by means of the adjustment of the shut-off element, which is, in particular, arranged in the region where the additional duct branches off from the induction duct and which permits an adjustment of the air flow both through the additional duct and through the induction duct directly into the compressor inlet. As a departure from the prior art, no additional pressure reservoirs and no units generating compressed air are necessary in this embodiment so that the design is substantially simplified.
For the case where, in the lower load range, the required boost pressure determined in a closed-loop and open-chain control unit falls below a threshold value, the shut-off element is adjusted into a position opening the additional duct and reducing, in particular shutting off, the air supply to the compressor inlet. This achieves the effect that, even in the low-load range, the rotor rotational speed of the exhaust gas turbocharger is raised to such an extent that the compressor can generate an appreciable compressor output and the rotational speed can be sufficiently raised within a short time for the desired build-up of boost pressure to be realized. The supercharger rotational speed is held at a comparatively high level, even at low-load operating points, so that the transient behavior of the exhaust gas turbocharger is clearly improved; in addition, the supercharger rotational speed range between low-load operating points and high-load operating points is reduced. This makes it possible to employ exhaust gas turbochargers of larger design and correspondingly increased mass moment of inertia; these exhibit design and thermodynamic advantages and are, in particular, characterized by an improved efficiency, by which means the consumption and emission behavior of the internal combustion engine is also improved.
In a preferred embodiment, the air-driven turbine and the compressor form a common component, the additional duct, which branches off from the induction duct upstream of the compressor inlet, opening into the compressor inlet level with the compressor impeller. This design embodiment makes it possible, without reversal of the direction of rotation of the supercharger rotor, to employ the compressor as an air-driven turbine at certain operating points of the internal combustion enginexe2x80x94low-load and/or small required boost pressurexe2x80x94because, due to the different supply of air to the turbine impeller in the compressor, different pressure relationships are created which permit use either as a compressor or as a turbine. In the low-load ranges, it is possible to realize a pressure drop across the compressor in which a higher pressure is present at the compressor inlet than at the compressor outlet. Because of this pressure drop, throttling is achieved across the compressor (or air turbine) and this, in low-load ranges, is sufficient to generate a necessary induction depression. This, as appropriate, makes it possible to dispense with a throttle butterfly, for adjusting the induction pipe pressure, in the induction pipe of the internal combustion engine.
In the compressor, it is expedient to introduce a throttle device in the compressor inlet in the region where the additional duct emerges, which throttle device supports the formation of a pressure drop between the opening of the additional duct into the compressor inlet and the compressor outlet, this permitting operation of the compressor as an air-driven turbine.
In a preferred embodiment, the throttle device is a variably adjustable guide vane cascade, which is adjustably configured in the opening region between the additional duct and the turbine or compressor inlet. This corresponds to an embodiment of a turbine with variable turbine geometry. In the operating range below the threshold value of the required boost pressure, the opening cross section can be adjusted, by altering the position of the guide vane cascade, to set the desired required boost pressure in the induction path of the internal combustion engine. Adjustable guide vanes are advantageously allocated to the guide vane cascade, an adjustable shut-off element being provided in the opening region, in accordance with a further advantageous embodiment, in particular an axially displaceable matrix with recesses for accommodating the guide vane cascade and by means of which complete closure of the opening cross section is possible.
In the embodiment as a shut-off element with recesses for accommodating the guide vane cascade, an additional degree of freedom is provided with respect to the adjustment of the opening cross section, it being possible, in the axially displaceable embodiment of the shut-off element, to adjust the inlet location of the additional duct into the compressor inlet axially with respect to the compressor impeller. By this means, particularly for the case where the compressor is simultaneously the air turbine, the incident flow of the induction air onto the rotor, and therefore the turbine behavior of the compressor, can be influenced.
As an alternative to or additionally to the embodiment where compressor and air turbine form a common component, it is also contemplated to embody the air turbine as an autonomous component independent of the compressor. This has the advantage that use can be made of substantially conventional turbine types which are only connected to the compressor impeller in the direction of rotation.
In accordance with a further advantageous feature of preferred embodiments, the induction air to be supplied to the air turbine is first preheated in a heat exchanger, by which means the pressure level upstream of the turbine rotor is increased and the pressure drop over the rotor of the air turbine is increased.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.